1. Field of the Invention
Embodiments of the present invention generally relate to the field of well drilling, particularly to the field of well drilling for the extraction of hydrocarbons from sub-surface formations, wherein the drill string is used as the well casing.
2. Description of the Related Art
The drilling of wells to recover hydrocarbons from subsurface formations is typically accomplished by directing a rotatable drilling element, such as a drill bit, into the earth on the end of tubing known as a “drill string” through which drilling mud is directed to cool and clean the drilling face of the drill bit and remove drilled material or cuttings from the borehole as it is drilled. After the borehole has been drilled or bored to its desired depth and location, the borehole is typically cased, i.e., metal tubing is located along the length of the borehole and cemented in place to isolate the borehole from the surrounding earth, prevent the formation from caving into the borehole, and to isolate the earth formations from one another. The casing is then perforated at specific locations where hydrocarbons are expected to be found, to enable their recovery through the borehole.
It is known to use casing as the drill string, and, when drilling is completed to a desired depth, to cement the casing in place and thereby eliminate the need to remove the drill string from the borehole. However, when casing is used in place of the drill string, any equipment or tooling used in the drilling of the well must be removed from the interior of the casing to allow an additional, smaller diameter casing and drill bit to drill the borehole further into the earth. Thus, the drill bit or drill shoe located at the end of the drill string must be eliminated as an obstacle, without pulling the casing from the borehole. Removal of the drill shoe is typically accomplished by drilling through the drill shoe with a second drill shoe or drill bit extended into the previously cemented casing, and thence into the earth beyond the just drilled drill shoe. Thus the drill shoe needs to be configured of a drillable material, which limits the loading which can be placed on the drill shoe during drilling and thus limits the efficiency of drilling with the drillable drill shoe. Typically a “drillable” drill shoe is configured of a relatively soft metal, such as aluminum, with relatively hard inserts of materials such as synthetic diamond located thereon to serve as the cutting material. Additionally, although the main body of the drillable drill shoe is configured of a readily drilled material, the hard cutters of the drill shoe tend to cause rapid wear and physical damage to the drill shoe being used to drill through the previous drill shoe, thus reducing the life of the drill bit, and thus the depth of formation the drill shoe can penetrate before it too must be drilled through by an additional drill shoe directed through the casing.
It is also known to provide a drill shoe having a relatively soft metal body, within which a plurality of stronger metal blades are received, upon which blades are supplied the cutters for cutting into the earth as the borehole progresses and which blades may be moved out of the area through which the drill shoe is drilled and subsequent casing penetrates, as is disclosed in U.S. Pat. No. 6,443,247, assigned to the assignee of the present invention and incorporated by reference herein in its entirety. This drill shoe includes an integral piston assembly therein, which, upon actuation by a drilling operator, pushes through the drill shoe and physically presses the harder metal blades, with the cutters thereon, into the annular area and/or the adjacent formation and out of the area through which the next drill shoe will pass. Thereafter, an additional drill shoe is passed down the existing casing to remove the remaining, relatively soft, metal mass of the drill shoe, and into the formation beyond the just drilled through drill shoe. Although this drill shoe configuration solves the problem encountered when the drill shoe would otherwise need to engage and grind up hard metal parts, the drill shoes still suffer from limited lifetimes because the blades will extrude or otherwise become separated from the relatively soft metal body of the drill shoe if the loading thereon exceeds a certain threshold. Thus, although this style of drill shoe has gained a high degree of commercial acceptance, the capability of the drill shoe remains limited.